From: James Rogers (jamesr@best.com)
Date: Thu Jan 09 1997 - 15:46:55 MST
At 12:41 PM 1/9/97 -0500, Michael Lorrey wrote:
>While some people have postulated the possiblity of life based on
>silicon, it simply does not have the same number of possible valence
>combinations as carbon does. While silicon life may form in some carbon
>poor regions of the universe (unlikely that such a state could exist,
>but not impossible), anywhere where carbon is in its normal abundance,
>and temperatures exist between -20 C up to 150 C, and there is water,
>life will form around carbon. Any silicon based primitive life forms
>will quickly be pushed out of the competition for resources with its
>carbon cousins because of carbon's greater ability to form complex
>molecules.
>
Although you are correct in most respects, you are exhibiting a common human
bias towards water-based chemistry. In much the same way people adopted a
decimal numbering system, most people (even many chemists) adopt a
water-based chemical view. Fact is, the choice of base solvents can be
pretty much arbitrary. The influence of water on us and our world tends to
creep into our assumptions about life on other worlds.
Evidence of this can be seen in most university inorganic chemistry classes.
Everyone learns inorganic chemistry under the assumption that water is the
solvent. At the lower class levels, they don't even suggest that there is
an entire parallel universe of chemical interactions based on, say, an H2S
solvent environment.
That said, I must say that water *is* a good solvent for life environments,
but it is by no means an exclusive club.
>Furthermore, any possible life forms developing on planets vastly
>dissimilar to our own would of neccessity have vastly different goals,
>resource needs, and would not compete with us for planets, as what we
>would find optimum would not neccessarily be optimum for such vastly
>different life forms, so we would probably have little interaction
>outside of information trade, assuming that we could even communicate.
In the situation you describe above, we might not be competing for living
space, but we would be competing for resources. A rich palladium deposit is
valuable whether you breathe oxygen or methane, and technology should allow
us to retrieve these materials from almost any environment.
>So now we know that an intelligent life form on another earthlike world
>will most likely be a carbon based biped. We can also infer that due to
>efficeincy/performance of optical/auditory arrangments, that the biped
>will have two eyes, as well as two ears. Will it see in our visible
>range? That depends on the spectra of the sun around which the planet
>orbits, but there must not be a high level of UV, as UV breaks down DNA
>rapidly and kills microorganisms, esp. the plankton. Given the spectra
>that an oxygen/nitrogen atmosphere, with a protective EM radiation belt,
>allows through, the peak output of a near sunlike star at planetary
>surfaces will be in the visible and near visible range. Evolution will
>take advantage of that, and eyes will develop sensitivity to whatever is
>most efficient, i.e. the visible range. Since two eyes is the minimum
>needed for stereoscopic vision, evolution willnot waste time with a
>third eye, and likewise for ears.
I don't agree with your assumption of two eyes and ears. In most organisms,
the number of sensors is a tradeoff between necessity, capability, and
processing power. There is no requirement for a minimum number of
auditory/optical sensors. Especially in the case of auditory sensors,
having more increases your auditory capabilities significantly.
Technically, you could detect and process surround sound with a single ear.
However, this would require considerably more processing power be devoted to
processing the signal. The fact that we have two ears eases the processing
requirements significantly. The processing load for humans could be
significantly lessened if we had additional ears. For example, humans have
poor front-back discrimination. The processing used by the brain to
accomplish this with only two ears amounts to a complex spectral analysis in
addition to other sensory queues, such as visual. A third ear would
substantially minimize the need for this type of processing, and a fourth
ear (poor up-down discrimination, too) would eliminate it all together. But
apparently evolution thought it was "good enough" in our case and had
processing power to spare. The directional ears found on many animals are
ways of combatting the discrimination limits of two ears without increasing
the brain requirements. On an alien world, it would be hard to ascertain
what the evolutionary path may be.
I generally agree that eyes are most likely to be a single pair, but this is
by no means certain either. There are many species of lower animals with
many eyes, and in some predator rich environments, it might be advantageous
to have more than a pair of eyes. The obvious drawback is that many good
eyes require significant quantities of processing power to go with each of
them, which is the reason I believe they did not commonly evolve, especially
in higher animals.
You are correct in your assessment of the visible wavelength range, but I
wanted to add a point or two. Many animals, including the higher animals,
can see into the UV range, and for some animals the visible range goes into
the infrared as well. There are good chemical reasons for vision in these
ranges. It should also be pointed out that as the frequency gets lower,
sensory resolution tends to degrade. And besides, sensitivity to microwave
frequencies, for example, would be nearly useless to most animals.
-James Rogers
jamesr@best.com
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